Cumene is isopropylbenzene (C6H5CH(CH3)2), and it is used as an important intermediate material in a variety of chemical industries, polymer industries, etc. At present, most of the cumene (isopropylbenzene) being produced is used for the preparation of phenol, acetone, etc.
Cumene is generally produced by reacting benzene and propylene under liquid or gas phase conditions in the presence of a catalyst. Technologies related to the preparation of cumene are proposed in Korean Unexamined Patent Application Publication No. 10-2011-0082160 and Korean Unexamined Patent Application Publication No. 10-2013-0008595, etc.
Cumene is mostly commercially prepared through an alkylation reaction and a trans alkylation reaction. Accordingly, an apparatus for preparing cumene includes an alkylation reaction unit and a trans alkylation reaction unit.
In the alkylation reaction unit, benzene and propylene react to produce cumene (isopropylbenzene) and, as a by-product, polyisopropylbenzenes (PIPB) such as diisopropylbenzene (DIPB), triisopropylbenzene (TIPB), etc. are produced through a reaction between cumene and propylene. The competitive reaction in the preparation of cumene is a polyalkylation reaction. In other words, it is a side reaction which produces the above-described PIPBs such as DIPB, TIPB, etc.
The trans alkylation reaction unit is used to react polyalkylated benzene, which is the PIPB, etc. produced through the above-described side reaction, with benzene to produce additional cumene.
Also, in addition to the substances described above, light materials (lights) such as C3 (propylene, propane, etc.), etc. and heavy materials (heavies), which are heavier than PIPB, are produced as additional products during the preparation of cumene, and along with these materials, unconsumed benzene, water, etc. are present. Therefore, in the alkylation reaction unit and trans alkylation reaction unit, lights such as C3 (propylene, propane, etc.), etc., PIPB, unconsumed benzene, water and other heavies, etc. are discharged in addition to the cumene (isopropylbenzene) of interest; these materials are either removed or recycled through a purification process in pursuit of high purity cumene.
In general, three distillation columns are used in the purification process of cumene. FIG. 1 is a diagram illustrating the configuration of an apparatus for purifying cumene according to the prior art. Referring to FIG. 1, the purification process for cumene according to the prior art can be schematically illustrated as follows.
The apparatus for purifying cumene is generally installed in connection with the above-described alkylation reaction unit and trans alkylation reaction unit, and includes 3 distillation columns such as a first distillation column, a second distillation column and a third distillation column.
The first distillation column is a benzene column 1 which recovers benzene from streams from the alkylation reaction unit and trans alkylation reaction unit.
In this case, an in-put line 1b which takes in a stream discharged from the alkylation reaction unit and an in-put line 1c which takes in a stream discharged from the trans alkylation reaction unit are connected to the front end portion of the benzene column 1. Also, an in-put line 1a through which fresh benzene flows in is connected to the front end portion of the benzene column 1. In addition, lights such as C3, etc. and water are discharged from the upper portion of the benzene column 1 through a lights out-put line 1d, whereas a cumene stream is discharged from the lower portion through a cumene stream out-put line 1e. Further, benzene is discharged from substantially the center of the benzene column 1 through a benzene recycle line 1f, and the discharged benzene is recycled.
The second distillation column is a cumene column 2 which recovers cumene from the cumene stream discharged from the lower portion of the benzene column 1.
In this case, cumene is discharged from the upper portion of the cumene column 2 through a cumene out-put line 2a and recovered. Also, from the lower portion of the cumene column 2, a PIPB stream is discharged through a PIPB out-put line 2b. 
The third distillation column is a PIPB column 3 which takes in and recycles the PIPB stream discharged from the lower portion of the cumene column 2.
In this case, PIPBs such as DIPB, etc. are discharged from the upper portion of the PIPB column 3 through a PIPB out-put line 3a and recycled. Also, from the lower portion of the PIPB column 3, heavies are discharged through a heavies out-put line 3b. 
Cumene (isopropylbenzene) of interest can be purified to a high purity and recovered through a purification process such as the above. In addition, energy is consumed in the above-described purification process. To each of the columns 1, 2 and 3, a heat source is provided for the separation of substances by the differences in boiling points, and most of the energy is consumed in such a separation process. In FIG. 1, reference numeral C represents a condenser, and reference numeral B represents a heat exchanger (or reboiler) for supplying heat.
However, the cumene purification process according to the prior art requires a large consumption of energy. As described above, each of the columns 1, 2 and 3 is provided with a heat source for the separation of substances, and without an efficient use of the heat sources being reviewed, the amount of energy consumed is large especially in such a separation process.